Passive Interferometric Detection of Ultrasound with a Large Dynamic Range

Optical detection of ultrasound often uses high Q-factors resonators to maximize sensitivity, compromising
the linear range of the scheme, making it more susceptible to external perturbations and incapable of
measuring strong acoustic signals. In this work, a passive pulse interferometry (P-PI) scheme was developed
for high dynamic-range measurements beyond the linear range of conventional interferometric techniques.
The general scheme of pulse interferometry composes of a wide-band pulse laser connected to an optical
resonator, whose output was connected to an unbalanced Mach-Zehnder interferometer (MZI) implemented
in PM fibers. The birefringence of PM fibers was utilized to detect phase shifts in a MZI that was not locked
to quadrature. In this scheme, the output of the resonator was connected to a dual-polarization MZI, in
which by careful selection of the lengths of each segment in the interferometer, the outputs represent
those of a 90° hybrid.
The performance of P-PI was tested and compared to conventional interferometric techniques for different
pressure levels. The signals measured with P-PI show a linear dependency between the peak-to-peak values
of the over a range of over 4 rad. For comparison, using conventional interferometric techniques results in
folding of the signal at high pressure levels - incompatible with high dynamic-range measurements.
In conclusion, we demonstrated P-PI ultrasound detection scheme that is capable of maintaining a linear
response under large pressure levels outside the linear range of conventional interferometric techniques
without compromising the sensitivity for small signals. Accordingly, P-PI extends the applicability of pulse
interferometry for ultrasound detection to scenarios in which a high dynamic range is needed. In addition,
all the components in our scheme may be fabricated in photonic circuits, making it scalable.